The present invention relates to magneto-optical data recording systems, and more particularly to such systems which are capable of overwriting new coded data on old data.
At present, most magneto-optical data recording systems erase old data and then write new data, so that the disc used must be rotated at least twice, once for erasing the old data from the disc and once for writing new data on the disc. This is because it is difficult to directly overwrite new data on the old data.
For example, in Japanese Patent Publication JP-A-52-23318, data is recorded on a thin magneto-optic layer by the interaction of a laser and a magnetic coil for generating a magnetic field. Namely, a magnetic domain is formed at a position heated by the laser beam using a magnetic field lower than magnetic coercive force in the thin layer. In this arrangement, a pulsated modulated laser beam and a direct current magnetic field are used corresponding to the coded data. According to this method, new data cannot be directly written so long as old data remains.
In Japanese Patent Publication JP-A-51-107121, a modulated magnetic field pulsated with coded data and a direct current laser beam are used. According to this method, new data can be directly overwritten on a disc where old data remains. However, in this method, the laser beam is irradiated onto the recording film with a constant light power, so that the recording film is overheated for longer time and the shape of the formed magnetic domain is not circular, but crescent. This method is problematic in signal processing.
Further, in Japanese Patent Publication JP-A-62-252553, a high frequency self-oscillating magnetic field generated by a high frequency self-oscillating circuit and a modulated laser beam pulsated with coded data are used. Since this method uses the high-frequency self-oscillating magnetic field, new limited data can be directly overwritten on a disc where old data remains, but any new data cannot be overwritten directly and the shape of the formed magnetic domain does not become circular, but crescent. Thus this method is also problematic in signal processing.
These conventional techniques are not suitable for overwriting any new data on a magneto-optic recording medium where old data remains recorded. Further, the shape of the formed magnetic domain would become crescent. There are also problems in the signal processing.